1. Field of the Invention
The present invention relates to a distributed feedback semiconductor laser suitable as a light source for long distance and large capacity optical data communication, and a method for producing the same.
2. Description of the Related Art
Recently, distributed feedback semiconductor lasers (hereinafter, referred to as "DFB lasers") have been put into practical use as light sources for long distance and large capacity optical data communication and multi-channel video data transmission such as CATV. The DFB lasers emit light having a single wavelength and thus have such advantages as high response speed and low noise. As a result of such advantages, the DFB lasers are in wide use as light sources for optical data communication. Two methods for causing distributed feedback of light have been theoretically shown by, for example, Kogelnik ("Coupled-Wave Theory of Distributed Feedback Lasers", Journal of Applied Physics, vol. 43, page 2327, 1972).
One of the methods is a refractive index coupled system, by which the semiconductor laser is structured so that the refractive index periodically changes in the cavity length direction, and generates laser oscillation having a wavelength corresponding to the period of change (Bragg wavelength and the vicinity thereof). A great number of DFB lasers produced by this method have been reported because of ease in performing the method. However, in a DFB laser produced by this method, laser oscillation theoretically occurs in either one of the two laser oscillation modes which interpose the Bragg wavelength and thus involves a high possibility that laser oscillation occurs in both of the two oscillation modes.
The other method is a gain coupled system, by which the semiconductor laser is structured so that the gain periodically changes in the cavity length direction, and generates laser oscillation having a wavelength corresponding to the period of change (Bragg wavelength). In a DFB laser produced in this method, oscillation theoretically occurs only at the Bragg wavelength and thus has a high possibility that laser oscillation has a single wavelength. However, a DFB laser having satisfactory characteristics has not been produced since the advent of the theory because of difficulties associated with performing the method.
Recently, a method for producing a DFB laser has been proposed whereby the gain is periodically changed by providing an absorption layer periodically in the semiconductor laser to generate satisfactory laser oscillation ("Long-Wavelength InGaAsP/InP Distributed Feedback Lasers Incorporating Gain-Coupled Mechanism", Photonics Technology Letters, vol. 4, page 212, 1992). FIG. 18 is a cross sectional view of a DFB laser having such a structure. N-type InGaAsP absorption layers 23 are buried periodically between n-type InP layers 21, 24 and 25, and an n-type InGaAsP optical waveguide layer 26, an active layer 29, a p-type InGaAsP optical waveguide layer 30, and a p-type InP cladding layer 31. A bandgap energy of the n-type InGaAsP absorption layer 23 is set to be smaller than the emission energy from the active layer 29. Accordingly, the n-type InGaAsP optical waveguide layer 26 absorbs emission from the active layer 29 periodically to cause a periodical change in the gain. Thus, there is a high possibility that a laser oscillation having a single wavelength is produced.
A method for producing the DFB laser shown in FIG. 18 will be described with reference to FIGS. 19A, 19B and 19C.
As is shown in FIG. 19A, the n-type InGaAsP absorption layer 23 and an n-type InP passivation layer 24 are grown sequentially on an n-type InP substrate 21 by a first step of crystal growth. Then, as is shown in FIG. 19B, prescribed areas of the n-type InGaAsP absorption layer 23 are etched to form a diffraction grating 22 having a plurality of areas of the n-type InGaAsP absorption layers 23 arranged periodically. Then, as is shown in FIG. 19C, an n-type InP cladding layer 25 is deposited by a second step of crystal growth to bury the n-type InGaAsP absorption layer 23.
This method requires etching the n-type InGaAsP absorption layer 23 once. A surface of the n-type InGaAsP absorption layer 23 exposed by etching is subjected to heating by the second step of crystal growth. Because of such heating, the exposed surface can be undesirably defected which degrades not only the optical characteristics of the n-type InGaAsP absorption layer 23, but also the reliability of the DFB laser regarding the performance over a long period of time.
The "wavelength chirp", which is generated by direct modulation of the semiconductor laser is a serious problem associated with long-distance and large capacity data communication. For further improvement in the data transmission characteristics, a light source having a lower level of wavelength chirp is desired.